Question

How many grams of phosphorus are in a sample of Ca3(PO4)2 that contains 74.0 g of calcium?

mass phosphorus = _______g

Answer 1

we know that

moles = mass / molar mass

so

moles of Ca = 74 / 40

moles of Ca = 1.85

now

consider Ca3(P04)2

we can see that

1 mole of Ca3(P04)2 contains

1) 3 moles of Ca

2) 2 moles of P

3) 8 moles of O

so

moles of Ca = 3 x moles of Ca3(P04)2

so

1.85 = 3 x moles of Ca3(P04)2

moles of Ca3(P04)2 = 0.61666

now

moles of P = 2 x moles of Ca3(P04)2

so

moles of P = 2 x 0.61666

moles of P = 1.233333

now

mass = moles x molar mass

so

mass of P = 1.23333 x 31

mass of P = 38.233

so

38.233 grams of Phosphorus is present